Continuity mistake: Just before the final play of the game starts there is a shot of the scoreboard. The scoreboard reads 4th and 10 (yards) with 10 seconds to go. The announcer then says it is 4th and 1 with 10 seconds to go.
Continuity mistake: When the offensive line is first shown, as they sit on the bench, the shot is of them from behind. In that shot the team jersey numbers are in this order 89, 46, 8, 66, 1. Then the camera view changes to be in front of the players only now going from the players left to their right is 99, 46, 8, 66, 89.
Audio problem: At the hospital Clark and Jason talk about Clark possibly being on steroids. Watching Jason's mouth reveals he actually says "and a college career." before he says it out loud a few seconds later.
Continuity mistake: Chloe makes an $80 bet with Mxyzptlk at the beginning of the episode. When he pays her in the Torch office, he peels off seven 20-dollar bills for a total of $140 instead of the $160 it should be. (00:09:00)
Continuity mistake: When Clark is in the barn throwing footballs through the tire. He starts out with four footballs on the rack, but after he throws two, there's only one ball left.
Answer: The kryptonite only creates mutations when it reacts with the environment and an individual. Remember, the FDA and several environmental groups did check out Smallville after the meteor shower, but found no danger in the rocks. Obviously they're not always emitting mutating rays, or all the flora and fauna of the surrounding area would be horribly mutated.
Phoenix